12-15-2010
With VM, low memory is excess paging. Memory only shows as free because a process exited and released their swap, or the system was recently rebooted, and so indicates nothing. If a process mmap's files, like dynamic libraries, those pages are still in use for that even though not assigned to any process! When no memory is free and you need a page, it just takes the least used and unmodified page from whoever was using it, and if anyone references that page, it will take a RAM page from some other VM page and read the requested VM page back in to it.
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LEARN ABOUT REDHAT
munlock
MUNLOCK(2) Linux Programmer's Manual MUNLOCK(2)
NAME
munlock - reenable paging for some parts of memory
SYNOPSIS
#include <sys/mman.h>
int munlock(const void *addr, size_t len);
DESCRIPTION
munlock reenables paging for the memory in the range starting at addr with length len bytes. All pages which contain a part of the speci-
fied memory range can after calling munlock be moved to external swap space again by the kernel.
Memory locks do not stack, i.e., pages which have been locked several times by calls to mlock or mlockall will be unlocked by a single call
to munlock for the corresponding range or by munlockall. Pages which are mapped to several locations or by several processes stay locked
into RAM as long as they are locked at least at one location or by at least one process.
On POSIX systems on which mlock and munlock are available, _POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the value PAGESIZE from <lim-
its.h> indicates the number of bytes per page.
RETURN VALUE
On success, munlock returns zero. On error, -1 is returned, errno is set appropriately, and no changes are made to any locks in the
address space of the process.
ERRORS
ENOMEM Some of the specified address range does not correspond to mapped pages in the address space of the process.
EINVAL len was not a positive number.
CONFORMING TO
POSIX.1b, SVr4
SEE ALSO
mlock(2), mlockall(2), munlockall(2)
Linux 1.3.43 1995-11-26 MUNLOCK(2)